This double-blind randomized, parallel group study will evaluate the efficacy and safety of lenalidomide (Revlimid, CC-5013) in combination with rituximab (MabThera/Rituxan) in patients with relapsed or refractory follicular lymphoma or marginal zone lymphoma. Patients will be randomized to receive either lenalidomide or placebo for twelve 28-day cycles in combination with rituximab. Anticipated time on study treatment is 1 year.